Design and Optimization of Thermal Systems Episode 2 Part 4 doc

Numerical Modeling and Simulation 297 may be coupled with the modeling of fluid flow adjacent to the rod and other parts of the system to complete the model of a given system 4.43 Consider heat conduction in a two-dimensional, rectangular region of length 0.3 m and width 0.1 m The dimension in the direction normal to this region may be taken as large The dimensionless temperature is given as 1.0 at one of the longer sides and as 0.0 at the others Solve the governing Laplace equation by the SOR method and determine the optimum relaxation factor Discuss how, in actual practice, such a simulation may be linked with those for other parts of the system 4.44 Consider the fan and duct system given in Example 4.7 Vary the zeroflow pressure, given as 80 in the problem, and the zero-pressure flow rate, given as 15 here, by 20% Discuss the results obtained Are they consistent with the physical nature of the problem as represented by the equations? 4.45 Show the information-flow diagram for Problem 4.18 Also, draw the information-flow diagram for the simulation of Problem 4.35 Do not solve the equations; just explain what approach you will use Acceptable Design of a Thermal System: A Synthesis of Different Design Steps 5.1 INTRODUCTION In the preceding chapters, we discussed the main aspects involved in the design of a thermal system An acceptable or satisfactory design must satisfy the given requirements for the system and must not violate the limitations or constraints imposed by the application, materials, safety, environmental effects, and other practical considerations At this stage, we are not concerned with the optimization of the system and are largely interested in obtaining a feasible design Though any design that meets the given requirements and constraints may be adequate for some applications, it is generally desirable to seek a domain of acceptable designs from which an appropriate design is selected on the basis of cost, ease of fabrication, availability of materials, convenience in usage, etc The various considerations that are involved in the development of an acceptable design of a thermal system have been discussed in Chapter These led to the following main steps: Formulation of the design problem Conceptual design Initial design Modeling of the system Simulation of the system Evaluation of the design Selection of an acceptable design Optimization of the design follows the determination of a domain of acceptable designs and is not included here Most of the other aspects, particularly problem formulation, conceptual design, modeling, and simulation, were discussed in detail in the preceding chapters The first step in the foregoing list quantifies the design problem, and the second step provides the basic idea or concept to achieve the desired goals The remaining steps constitute what might be termed as the detailed, quantitative design process, or simply the design process for 299 300 Design and Optimization of Thermal Systems convenience These steps analyze the design and ensure that the problem statement is satisfied In this chapter, we will consider the synthesis of the different steps and stages that constitute the design effort in order to obtain an acceptable design Individual aspects, such as modeling and simulation of thermal systems, which were discussed in detail earlier, will be considered as parts of the overall design strategy The main purpose of this chapter is to link the different aspects that are involved in the design of a thermal system and to demonstrate the design procedure, starting with the problem formulation, proceeding through modeling and simulation, and ending with an acceptable design Examples are employed to illustrate this coupling Several diverse thermal systems, ranging from those in materials processing to those in energy and environmental systems, were considered in the previous chapters It has been shown that the basic concerns, modeling, simulation, and system characteristics, vary significantly from one class of systems to another For instance, lumped steady-state modeling is usually adequate for refrigeration and air-conditioning systems, leading to algebraic equations, whereas distributed time-dependent modeling is generally needed for manufacturing processes and electronic equipment cooling, resulting in partial differential equations Consequently, the simulation procedures vary with the type of system under consideration The design strategy itself may be affected by these considerations Therefore, examples of thermal systems from different application areas are considered in this chapter and the corresponding design strategies presented The systems considered range from relatively simple ones to fairly complicated ones in order to demonstrate the applicability of the basic ideas to the design of a wide variety of systems Before proceeding to the complete design process for typical thermal systems, an aspect that needs more detailed consideration is that of initial design In many cases, the initial design is reached by considering the requirements and constraints of the problem and choosing the design variables, through approximate analysis and estimates, so that these satisfy the given problem statement If different components are to be chosen and assembled for a thermal system, the choice of these components is guided by the requirements and constraints, so that the initial design is itself an acceptable design Though redesign is obviously needed in case the initial design is not acceptable, it is important to employ the best possible initial design so that it is either acceptable by itself or the number of redesigns needed to converge to an acceptable design is small 5.2 INITIAL DESIGN The search for an initial design follows the formulation of the problem and the conceptual design It is thus the first step in the quantitative design procedure The analysis of the system, through modeling and simulation, and evaluation of the design for its acceptability are based on the initial design The initial, starting design affects the convergence of the iterative design process and often Acceptable Design of a Thermal System 301 even influences the final acceptable or optimal design obtained Therefore, the development of an initial design is a critical step in the design procedure, and considerable care and effort must be exerted to obtain a design that is acceptable or as close as possible to an acceptable design Ideally, the design variables should be selected so that the initial design satisfies the given requirements and constraints Unfortunately, this is usually not possible for thermal systems because analysis only yields the outputs on system behavior for given inputs, rather than solve the inverse problem of yielding the inputs needed for a desired behavior If the outputs and inputs were connected by simple relationships that could be inverted to obtain the inputs for required outputs, the problem would be considerably simplified However, thermal systems usually involve complexities arising from nonlinear mechanisms, partial differential equations, coupled phenomena, and other complications, as discussed in Chapter This makes it very difficult to solve the inverse problem in order to select the design variables, in an initial design, to satisfy all the requirements and constraints Consequently, iteration is generally necessary to obtain a satisfactory design Several approaches may be adopted in the selection or development of the initial design The approach that is appropriate for a given problem is a function of the nature of the thermal system under consideration, information available on previous design work, and the scope of the design effort Some of the commonly used methods for obtaining an initial design are Selection of components to meet given requirements and constraints Use of existing systems Selection from a library of previous designs Use of current engineering practice and expert knowledge of the application Selection of Components In general, a combination of all the approaches given above is used to come up with the best initial design for practical thermal systems However, each of these may also independently yield the desired starting point for iterative design Selection of components is particularly valuable in thermodynamic systems, such as refrigeration, air conditioning, and heating systems, where the design of the overall system generally involves selecting the different components to meet the given requirements or specifications An example of this is the air-cycle refrigeration system, based on the reverse Brayton cycle and shown in Figure 5.1, which is commonly used aboard jet aircrafts to cool the cabin The turbine, the compressor, and the heat exchanger may be selected based on the desired temperature and pressure in the cabin, along with the thermal load, to obtain an initial design An analysis of the thermodynamic cycle shown yields the appropriate specifications of the components for an ideal cycle or for a real one with given isentropic efficiencies (Reynolds and Perkins, 1977; Howell and Buckius, 1992) For an ideal 302 Design and Optimization of Thermal Systems Heat input Heat exchanger 2a PH Compressor Turbine Work PL Heat exchanger 4a Heat rejected (a) PH Turbine Temperature Heat input Actual 2a 2s PL Compressor Actual 4s 4a Heat rejected Entropy (b) FIGURE 5.1 The hardware and the thermodynamic cycle, with real, nonideal compressor and turbine, for the Brayton cycle Acceptable Design of a Thermal System 303 cycle, the efficiency , which is the ratio of the work done to the energy input into the system, is given by the expression 1 PH PL (5.1) where is the ratio of the specific heat at constant pressure Cp to that at constant volume Cv, and PH, PL are the high and low pressures in the system, respectively The corresponding temperatures can be calculated for isentropic processes and then for a real, nonideal system using the efficiencies Any constraints on pressures or temperatures given in the problem can be taken care of by a proper choice of these components A given range of desired efficiency for satisfactory systems may also be taken as a requirement Therefore, the initial design itself satisfies the problem statement and is an acceptable design This design may be modeled and simulated to study the system behavior under different operating conditions to ensure satisfactory performance in practical use Example 5.1 and Example 5.2 discuss this approach for thermodynamic systems Existing Systems The development of an initial design based on existing systems for applications similar to the one under consideration is a very useful technique Unless a completely new concept is being considered for the given application, systems that perform similar, though different, tasks are usually available and in use For instance, if a forced-air furnace is being designed for continuous heat treatment of silicon wafers as a step in the manufacture of semiconductor devices, as shown in Figure 5.2, similar systems that are being used for other processes, such as baking of circuit boards and curing of plastic components, may be employed to obtain initial estimates of the heater characteristics, wall material and dimensions, conveyor design, interior dimensions, etc This provides the starting point for the iterative design-redesign process, which varies the relevant design variables to arrive at an acceptable design Heaters Silicon wafers Conveyor FIGURE 5.2 A thermal system for the heat treatment of silicon wafers in the manufacture of electronic circuitry 304 Design and Optimization of Thermal Systems Library of Previous Designs Any industry involved with the design of systems and equipment would generally develop many successful designs over a period of time for a variety of applications and design specifications Even for the design of a particular system, several designs are usually generated during the process to obtain the best or optimal design Consequently, a library of previous successful designs can be built up for future use Note that these designs may not have been translated into actual physical systems and may have remained as possible designs for the given application Such a library provides a very useful source of information for the selection of an initial design For instance, an effort on the design of heat exchangers would give rise to many designs that may not be chosen for fabrication because they were not the optimum or because they did not meet the requirements for a given application However, for different design specifications, some of the earlier designs that were discarded might be satisfactory Similarly, the design of an air compressor may yield many designs that are discarded because the pressure or the flow rate is too low However, if this information is retained, it can be used for selecting an initial design for some other applications Therefore, considerable effort is saved in the development of the initial design if such a library of earlier designs, along with their specifications, is available As soon as a new design problem is initiated, the library may be employed to obtain a design with outputs as close as possible to the given requirements For instance, if the total rate of heat transfer desired from the heat exchanger is given, a design that gives the closest heat transfer rate is chosen from the design library This approach is particularly suitable for equipment, such as heat exchangers, heat pumps, boilers, and refrigerators Expert Knowledge The last approach for developing an initial design is based on information available on the particular application and corresponding types of thermal systems, along with current engineering practice Such an approach is very hard to quantify because the available information is often vague and may not have a solid analytical foundation This is what is often termed as expert knowledge, i.e., the information obtained from an expert in the area Several ideas developed over the years form the basis for such knowledge and play a major role in determining what is feasible Information from earlier problems and attempts to resolve them is also part of this knowledge Many aspects in thermal systems are very difficult to analyze or measure, such as contact thermal resistance between surfaces, radiative properties of surfaces, surface roughness, fouling in heat exchangers, and losses due to friction Similarly, random processes such as demand for power, changes in environmental conditions, and fluctuations in operating conditions are not easy to ascertain In all such circumstances, current engineering practice and available information on the given application are used to come up with the initial design These aspects are considered in greater detail in terms of knowledgebased design methodology in Chapter 11 Acceptable Design of a Thermal System 305 Example 5.1 A refrigeration system is to be designed to maintain the temperature in a storage facility in the range of –15 to –5 C, while the outside temperature varies from 15 to 22 C The total thermal load on the storage unit is given as 20 kW Obtain an initial design for a vapor compression cooling system Solution Since the lowest temperature in the storage facility is –15 C, the evaporator must operate at a temperature lower than this value Let us select the evaporator temperature as –25 C Similarly, the ambient temperature can be as high as 22 C Therefore, the condenser temperature must be higher than this value to reject energy to the environment Let us take the temperature at which the condenser operates as 30 C The total thermal load is 20 kW, which is 20/3.517 5.69 tons Therefore, the refrigeration system must be capable of providing this cooling rate Since additional energy transfer may occur to the system and also for safe operation, let us design the system for 7.5 tons, which gives a safety factor of 7.5/5.69 1.32 We must now choose the refrigerant Because of environmental concerns with chlorofluorocarbons and because of the relatively large refrigeration system needed here, let us choose ammonia as the refrigerant The various parts of the system are shown in Figure 1.8(a) All these parts, except the compressor, usually have high efficiencies and may be assumed to be ideal The compressor efficiency could range from 60 to 80% Let us take this value as 65% The thermodynamic cycle in terms of a temperature-entropy plot is shown in Figure 5.3 The fluid entering the Temperature, T 2s 30°C Condenser 2a Compressor Throttling valve –25°C Evaporator Entropy, s 2a T(°C) –25 P(kPa) 151.5 Saturated vapor 188.7 1167.1 Superheated vapor 30 1167.1 Saturated liquid –25 151.5 Liquid-vapor mixture FIGURE 5.3 Thermodynamic cycle for the vapor compression refrigeration system considered in Example 5.1, along with the calculated conditions at various states 306 Design and Optimization of Thermal Systems throttling value is assumed to be saturated liquid and that leaving the evaporator is assumed to be saturated vapor These conditions are commonly employed in vapor compression refrigeration systems The nonideal behavior of the compressor is seen in terms of an increase in entropy during compression For ammonia, the various pressures may be determined from available tables or charts (Van Wylen et al., 1994) Therefore, the pressure at the inlet to the compressor is 151.5 kPa The pressure at the entrance to the throttling valve is 1167.1 kPa, which is also the pressure at the exit of the compressor The temperatures at the evaporator exit and valve entrance are –25 C and 30 C, respectively The enthalpy at the compressor exit is obtained from h2 s h1 h2 a h1 0.65 where is the compressor efficiency and the various states are shown in Figure 5.3 The entropy is constant between the states 2s and Using this condition, the enthalpy h2s is obtained as 1733 kJ/kg Therefore, with h1 1430.9 kJ/kg, h2a is obtained from the preceding equation as h2a 1895.7 kJ/kg This also gives the temperature at the compressor exit as 188.7 C The coefficient of performance (COP) is obtained as COP Heat removed Energy input h1 h4 h2 a h1 2.34 Also, the heat removal rate, per unit mass flow rate of the refrigerant, Q/m , is Q m h1 h4 1430.9 h3 1430.9 342.5 1088.4 kJ/kg assuming enthalpy to remain unchanged in the throttling process, i.e., h3 h Since the total required cooling rate is 7.5 tons 26.38 kW, the mass flow rate m of the refrigerant is 26.38 m 24.24 10 kg/s 87.25 kg/hr 1088.49 Therefore, an initial design for the refrigeration system is obtained It is seen that several design decisions had to be made during this process Clearly, different values of the design variables could have been chosen, leading to a different initial design This implies that the design obtained is not unique In addition, because each part was chosen to satisfy the given problem statement, the initial design itself is an acceptable design The fluid chosen is ammonia and the system capacity is 7.5 tons The inlet and outlet conditions for each system part are obtained in terms of the inlet temperature and pressure, as given in Figure 5.3 The mass flow rate of the refrigerant is 87.25 kg/h Thus, these items may be procured based on the given specifications Because the items available in the market may have somewhat different specifications, the design may be adjusted to use available items, rather than have these custom made, in order to reduce costs However, the system should be analyzed again if these items are changed to ensure that it meets the given requirements and constraints Acceptable Design of a Thermal System 307 Example 5.2 A remote town in Asia is interested in developing a 20 MW power plant, using the burning of waste material for heat input and a local river for heat rejection It is found that temperatures as high as 350 C can be attained by this heat source, and typical temperatures in the river in the summer are around 30 C Obtain a simple initial design for such a power plant Solution Temperature, T A Rankine cycle, such as the one shown in Figure 2.15, may be chosen without superheating the steam to simplify the system This system has been analyzed extensively, as given in most textbooks on thermodynamics, and can be designed based on available information (see Moran and Shapiro, 2000) Water is chosen as the working fluid, again because of available property data, common use in typical power plants, and easy access to water at this location Due to the temperature ranges given, the boiler temperature is taken as 300 C to ensure heating and boiling with energy input at 350 C The condenser temperature is taken as 40 C to allow heat rejection to the river water, which is at 30 C Then the initial temperature cycle of the proposed power plant may be drawn, as shown in Figure 5.4 The various states are given, with the idealized states indicated by subscript s, as in the previous example Now, we can proceed to first model the system and then analyze the thermodynamic cycle All the components are taken as lumped, in order to simplify the model and because interest lies mainly in the energy transport and not in the detailed information for each component The process is approximated as steady, which would apply for a steady operation of the power plant and not for the startup and shutdown stages or for power surges The transient effects, which considerably complicate the analysis, may be considered later for designing the control system Thus, the analysis with steady lumped components will lead to coupled algebraic equations, which can be solved to obtain the power delivered, water flow rate needed, heat input, and other desired quantities Boiler 300°C Turbine 4s Pump Condenser 40°C 2s Entropy, s FIGURE 5.4 Thermodynamic cycle for the power plant design considered in Example 5.2 308 Design and Optimization of Thermal Systems Considering first the ideal cycle with isentropic turbine and pump, the steam tables are used to obtain properties at the relevant temperatures We find that, for saturated steam, the enthalpy h1 2749 kJ/kg and entropy s1 5.7045 kJ/kg, which is equal to s2s for an ideal turbine Then the quality x2s is obtained as x2 s s2 s s f sg s f 5.7045 0.5725 8.2570 0.5725 0.6678 where the subscripts f and g refer to saturated liquid and gas, respectively This gives h2s hf x2s(hg – hf) 167.57 0.6678 2418.6 1782.71 kJ/kg Similarly, for saturated liquid, h3 167.57 kJ/kg, s3 s4s 0.5725 kJ/kg The enthalpy h 4s is obtained by using the ideal pump work per unit mass, v3(p4 – p3), where v3 is the specific volume at state and the p’s are the pressures Thus, h 4s h3 v3 (p4 – p3) 167.57 1.0078 167.57 8.64 176.21 kJ/kg 10 (8.581 – 0.007384) 103 where the pressures are in MPa and 103 is used to obtain the work in kJ/kg Then, the work done, or power output, for the ideal case is given by W m(WTurbine,ideal WPump,ideal ) m[( h1 h2 s ) ( h4 s h3 )] where m is the mass flow rate of water/steam It is calculated for the ideal cycle as m ( 20 MW )(1000 kW/MW ) 957.65kJ/kg 20.88kg/s We can now include the effect of turbine and pump efficiencies Taking them at typical values of 80%, we have h1 h2 h1 h2 s 0.8 which gives h2 as 1975.97 kJ/kg Similarly, the pump work becomes (WPump, ideal)/0.8 10.8 kJ/kg This then gives h4 167.57 10.8 178.37 kJ/kg These values can now be used to obtain the water flow rate as 26.24 kg/s The heat input is given by m (h1 – h4) 26.24 (2749.0 – 178.37) kW 67.45 MW The heat rejected at the condensers is m (h2 – h3) 47.45 MW The overall thermal efficiency is 20/67.45 0.2965, or 29.65% Thus, an acceptable initial design of the thermal system is obtained by choosing components and thermodynamic states based on given constraints and requirements The efficiencies of the turbine and the pump can be adjusted if better information is available As in Example 5.1, the design is not unique and several acceptable designs can be developed The various components, such as the turbine, Acceptable Design of a Thermal System 309 pump, condensers, and boiler, can be procured on the basis of the needed flow rate, pressure, and temperature ranges The sensitivity of the design to variations in the components can be studied in order to choose available items instead of custommade ones to reduce cost These two examples demonstrate the commonly used approach for developing an initial design from the given problem statement so that an acceptable design is obtained Once such an initial design is obtained, the operating conditions and component characteristics may be varied in the simulation to optimize the system, as discussed later 5.3 DESIGN STRATEGIES 5.3.1 COMMONLY USED DESIGN APPROACH A strategy that is frequently employed for the design and optimization of thermal systems was discussed in earlier chapters An initial design is developed based on the problem statement and the corresponding system is modeled, simulated, and evaluated If the given requirements and constraints are satisfied, the initial design is acceptable; otherwise, a redesign process is undertaken until an acceptable design is obtained Clearly, this particular strategy is not unique, even though this is the most commonly used approach because of the systematic flow of information and the ease of implementation In addition, as discussed earlier, the initial design may be based on existing systems and processes and thus result in a design that is very close to the final acceptable design However, other strategies have been developed and are used for a variety of applications Two strategies that are based on modeling and simulation are presented here 5.3.2 OTHER STRATEGIES Adjusting Design Variables A frequently used approach is based on using the analysis, which incorporates modeling and simulation, to study a range of design variables and determine the resulting outputs from the system for a typical, fixed set of operating conditions The basic concept is kept unchanged and the design variables, such as dimensions, specifications, and characteristics of components like fans, blowers, heaters, and pumps, geometrical configuration, and materials, are varied over their given ranges and the effect on the important quantities in the problem investigated The resulting relationships between the outputs and the inputs may also be expressed in terms of correlating equations, using the curve-fitting techniques presented in Chapter An acceptable design is then obtained by choosing the appropriate values for the various design variables based on the problem statement and quantitative simulation results Different Designs Another strategy considers a collection of chosen designs and employs modeling and simulation to study the system behavior over the expected range of operating 310 Design and Optimization of Thermal Systems Modeling and simulation Design variables Outputs Selection of variables for acceptable design Fixed operating conditions (a) Operating conditions Modeling and simulation Different designs System characteristics Selection of acceptable design (b) FIGURE 5.5 Two different strategies for design of a thermal system: (a) Using design variables as inputs for fixed operating conditions; (b) using operating conditions as inputs for different designs conditions An initial design is not the starting point and simulation results are obtained for a variety of designs An acceptable design is obtained from the various designs considered by comparing the simulation results with the problem statement, ensuring that all the requirements and constraints are satisfied Both of these strategies are shown schematically in Figure 5.5 The main difference between these and the approach discussed in detail earlier (Figure 2.13) is that an initial design is not the starting point for the design process Extensive simulation results are obtained for a range of design variables for fixed operating conditions in one case and for a variety of designs under different operating conditions in the other The desired acceptable designs are selected based on these results and the formulation of the design problem Examples Let us consider the ingot casting system shown in Figure 1.3 Suppose the system is to be designed to obtain a solidification time s smaller than a given value, without violating given constraints on temperature gradients in the materials The solidification time is typically the time taken to solidify a given volume fraction of the melt, say 80%, since the ingot may be removed from the mold at this stage without waiting for the entire liquid region to solidify A mathematical model and a simulation scheme may be developed for this process to compute the solidification time for different values of the design variables, keeping the molten material and dimensions of the enclosed region fixed A simple one-dimensional model Acceptable Design of a Thermal System 311 may be developed assuming negligible flow in the melt Then, the numerical results on how solidification proceeds with time for a range of design variables, such as the wall material and thickness and convective heat transfer coefficient (representing a fan or circulating water for cooling the mold) at the outer surface of the wall, may be obtained The governing equations for this simple model are (Viswanath and Jaluria, 1991) T T y2 Ts Ts y2 (5.3) Tm y2 (5.4) s Tm m where the subscripts , s, and m refer to the liquid, solidified region, and mold; is the thermal diffusivity; and y is the coordinate distance, as shown in Figure 5.6 Then numerical simulation may be employed to compute the location of the solid/liquid interface as a function of time, thus yielding the amount solidified Therefore, the time needed to solidify a given amount of material can be determined For two- or three-dimensional problems, the progress of solidification from different sides may be determined to obtain the volume of the solidified material, if the solidification in different directions is assumed to be independent Some of the typical results are shown in Figure 5.7 through Figure 5.9, indicating the effects of mold wall thickness d = Wm Wo, thermal conductivity of mold material km (normalized by ks, the thermal conductivity of the solid), and convective heat transfer coefficient h at the outer wall of the mold Thus, the effect of the different variables on solidification time s is obtained A larger mold wall thickness and thermal conductivity and a larger h all lead to faster solidification, Mold Solid Melt x y W Wo FIGURE 5.6 A one-dimensional model for solidification Wm 312 Design and Optimization of Thermal Systems 0.15 Solid thickness (m) 0.12 0.09 Tpour = 1850 K h = 80 W/m2 K Mold material Iron 0.06 Mold wall thickness d cm 2.5 cm cm 0.03 0.00 0.0 180.0 360.0 540.0 720.0 900.0 Time (s) FIGURE 5.7 Variation of the rate of solidification with the mold wall thickness dW 0.15 Solid thickness (m) 0.12 0.09 0.06 km/ks 0.03 1.0 0.4 0.2 0.00 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 Time (s) FIGURE 5.8 Effect of the thermal conductivity of the material of the mold on the rate of solidification Acceptable Design of a Thermal System 313 0.150 Solid thickness (m) 0.125 0.100 Tpour = 1850 K 0.075 d = 3.0 cm Mold material Iron 0.050 h values (W/m2K) h = 20 h = 40 h = 70 h = 100 0.025 0.00 0.0 220.0 440.0 660.0 880.0 1100.0 Time (s) FIGURE 5.9 Effect of the convective heat transfer coefficient h on the solidification rate as physically expected Similarly, different operating conditions, such as ambient temperature, initial temperature of the mold, and initial temperature of the melt Tpour may be considered for a group of different designs, specified in terms of the design variables From these results, an acceptable design may be obtained to achieve the desired solidification time s for solidifying 80% of the given volume More sophisticated models have been developed and analysis of this system has been carried out by several investigators in recent years because of the importance of solidification in many manufacturing processes Similarly, thermal systems arising from other application areas may be considered to illustrate the use of these two design strategies For instance, the stratified water thermal energy storage system discussed in Example 3.5 may be taken The simplified one-dimensional, vertical transport model yielded the governing equation W Z Z2 H (5.5) where all the terms in the equation were defined earlier and nondimensionalization was used to reduce the number of parameters Here, W and H are the dimensionless vertical velocity and convective heat transfer coefficient, respectively Therefore, the equation may be solved numerically for arbitrary values of the parameters W and H to yield the temperature distribution as a function of time Figure 5.10 shows the results obtained from such a simulation for a typical energy storage system Therefore, for given flow rate, inlet/outlet locations, and discharge FIGURE 5.10 Calculated temperature profiles in an enclosed body of water for two inflow/outflow configurations at different values of dimensionless time 314 Design and Optimization of Thermal Systems Acceptable Design of a Thermal System 315 temperature into the tank, the resulting temperature at the outlet may be calculated as a function of time If hot water is to be supplied for a given duration at a specified minimum temperature level, the system may be designed by varying the dimensions, insulation, outer surface cooling, etc., to meet this requirement The simulation is used to generate results for a range of design variables and operating conditions An acceptable design is then selected by comparing these results with the requirements and constraints Selection of Acceptable Design Extensive work has been done on the analysis of a wide variety of thermal systems, and sophisticated models and simulation results are often available in the literature However, the use of these results to obtain a satisfactory design is not a trivial exercise, even though most analyses claim that the results obtained will be valuable in design As mentioned earlier, analysis is much simpler than design because the outputs resulting from given inputs are to be determined In design, the inverse problem of finding the variables or conditions under which the desired outputs will be obtained is to be solved By generating extensive simulation results, the attempt is to solve the inverse problem for design by correlating the outputs with the inputs Certainly, it is necessary to focus on some important parameters in order to obtain an acceptable design from simulation results For instance, solidification time was taken as the main aspect in ingot casting The duration for which water can be supplied without its temperature going below a minimum value may be the criterion for a water energy storage system Then, such an output may be expressed in terms of the inputs by means of correlating equations, derived by the use of curve-fitting techniques If such expressions are available, the design problem becomes relatively simple because the conditions needed for satisfying the requirements may be calculated easily from these expressions In summary, different design strategies may be developed for different applications The systematic approach represented by Figure 1.4 and Figure 2.13 is the most commonly employed strategy because it is also often the most efficient one In most other approaches, extensive computations, which are generally timeconsuming and expensive, are used in order to generate the results from which the appropriate design is extracted It may also be mentioned here that, even though numerical simulation is used for most of the inputs needed for design, experimentation may also provide important data, particularly for cases where an accurate mathematical model is not easily obtained Example 5.3 A thermal system consisting of a solar collector and an energy storage tank with recirculating water, as shown in Figure 5.11, is to be designed to obtain 2.1 105 kJ of stored energy over a 10-hour day The ambient temperature is given as 20 C and 316 Design and Optimization of Thermal Systems So la rfl ux Convective loss Solar collector Energy storage tank Pump FIGURE 5.11 Solar collector and storage tank system considered in Example 5.3 the water temperature is initially at this value The water temperature in the storage tank must reach a value greater than 40 C, but less than 100 C, to be used in an industrial application The collector receives a constant solar flux of 290 W/m2 and loses energy by convection at a heat transfer coefficient h of W/(m2 K) to the ambient medium Obtain an acceptable design Solution A very simple mathematical model for this system is obtained by assuming that the convective heat loss qc from the collector can be approximated as qc hA To 20 20 where To is the maximum temperature attained over the day and A is the surface area of the collector, implying that an approximate average surface temperature is used to obtain the heat loss Actually, the temperature varies nonlinearly with time and a differential equation needs to be solved to obtain the temperature variation This approximation considerably simplifies the model In addition, the storage tank is assumed to be well mixed so that a uniform temperature exists across it Heat loss from the tank is neglected With these assumptions, an energy balance for the collector yields 290 To 20 20 A(10 3600) 2.1 105 103 where a constant heat flux input into the collector arises over a 10-hour period and both sides of the equation are in Joules An energy balance for the storage tank of volume V gives 1000 4200 V (To – 20) 2.1 105 103 Acceptable Design of a Thermal System 317 where the density of water is taken as 1000 kg/m3 and the specific heat at constant pressure as 4200 J/(kg K) The preceding two equations may be simplified to give [290 – 2(To To 20)] A 50 V 5833.3 20 Therefore, these equations may be used to calculate the collector area A and the volume V of the storage tank for an acceptable design The requirement of the total energy is already satisfied The only other requirement is that 100 To 40 C Therefore, a domain of acceptable designs can be generated with these limitations We may write these equations as V 50 To 20 and A 5833.3 290 100/V If To is chosen as 45 C, V is obtained as m3 and A as 24.3 m2 This gives an acceptable design because it satisfies the given requirements and constraints Similarly, if To is chosen as 95 C, V is 0.67 m3 and A is 41.66 m2 For To 70 C, V is m3 and A is 30.7 m2 Clearly, a unique solution is not obtained and an infinite number of designs can be generated in the domain given by the requirement 100 To 40 C If the system is optimized, with respect to cost or some other chosen criterion, this domain is substantially reduced, leading to an essentially unique solution in many cases This is a small thermal system and approximations are used to develop a simple mathematical model Models that are more complicated can easily be developed for greater accuracy However, this example illustrates a design strategy based on modeling and simulation, without using an initial design, to develop an acceptable design It also indicates the crucial need to optimize the system 5.3.3 ITERATIVE REDESIGN PROCEDURE Iteration is an essential part of design in most design strategies and procedures because an inverse problem is to be solved In the analysis of thermal systems, the effort is directed at obtaining the output characteristics for given inputs such as operating conditions and design variables However, in design, the requirements and constraints are given and the variables that result in a system that satisfies these are to be determined As a result, the solution to the problem is not unique and several designs may have to be considered before obtaining one that satisfies the requirements and constraints Convergence Any iterative procedure requires a criterion for convergence or termination of the iteration In the design problem, since the given requirements and constraints may involve several variables and thus many criteria for convergence, it is useful to focus on a particular quantity or condition that is of particular significance to 318 Design and Optimization of Thermal Systems the problem at hand This quantity may then be followed as iteration proceeds to ensure that the scheme is indeed converging and to stop the iteration when the desired results have been obtained or if a specified number of iterations have still not yielded a solution For instance, in a cooling system, the rate of heat removal may be chosen as the main quantity of interest, even though the flow rates and temperatures are also important in the design Similarly, the temperature of a material emerging from a heat treatment furnace may be selected as the criterion for following the iteration scheme Even though a particular parameter or quantity is considered with respect to the iteration scheme, the design obtained at convergence must be evaluated to ensure that all the design requirements and constraints are satisfied Since the quantity chosen for termination of the iteration is the most important aspect or a combination of dominant aspects in the design problem, there is a good possibility that the design obtained at convergence will be an acceptable design However, if it is not satisfactory, the design variables may be varied near the converged design to seek an acceptable design If, despite these efforts, a satisfactory design is not obtained, some of the requirements or constraints may have to be relaxed to obtain a solution If x1, x2, x3, , xn represent n quantities of interest in a thermal system to be designed, the requirements may be specified as xi di , xi di , or, xi di (5.6) which may be written as xi di 0, xi di 0, or, xi di (5.7) where any one of the preceding conditions may apply to a given quantity and di represents the given requirements, with i 1, 2, 3, , n The inequalities may be converted into equalities by assuming an acceptable tolerance level , as, for instance, xi – di , where may be positive or negative For example, in a heat exchanger, the given requirements relate to the flow rates, temperatures, and heat transfer rate Thus, if the inlet flow rate and temperature of the hot and cold fluids are fixed, the outlet temperature of the cold fluid as well as the heat transfer rate may be taken as the requirements, with the configuration, dimensions, and insulation of the heat exchanger as design variables If energy losses to the environment are included, the efficiency of the system may be defined as the ratio of the energy gained by the cold fluid to that lost by the hot fluid An efficiency greater than a given value may then be a requirement Several such requirements are generally associated with the design of a thermal system However, the most important requirement, say the outlet temperature of the cold fluid in the present example, may be chosen as the criterion for convergence of the scheme Acceptable Design of a Thermal System 319 If it is not possible to isolate a particular quantity for the iterative scheme, a combination of these variables or of their difference from the required values, such as Y x1 x2 x3 or Y (x1 d1) d2) (x2 (x3 d3) (5.8) may be chosen and the function Y employed to keep track of the progress of the iteration If both positive and negative values of the variables or of their differences from the requirements are considered, Y may be defined as Y |x1| |x2| |x3| or Y |(x1 x3 or Y (x1 d1)| d2)| |(x2 |(x3 d3)| (5.9) or as Y x1 x2 d1)2 (x2 d2)2 (x3 d3)2 (5.10) A square root of the expressions on the right-hand sides of the two equations given in Equation (5.10) may also be employed All the terms in the preceding equations for Y should generally be normalized by the required values, such as di, to make them of comparable magnitude Therefore, several different requirements may be included in a design parameter or quantity that is used to follow the iterative process and to determine its convergence For instance, in the case of the heat exchanger discussed previously, the design parameter Y may be taken in terms of the cold fluid outlet temperature To and heat transfer rate Q as Y To Tr Tr Q Qr Qr 1/ (5.11) where the subscript r refers to the required values Then the desired value of Y for the given problem may be determined, being zero if differences from the requirements are employed as in Equation (5.11) Weighting factors may also be used to accentuate the importance of certain requirements over the others Therefore, the iterative redesign process becomes quite similar to the iterative procedures employed for solving nonlinear algebraic equations, as outlined in Chapter The design parameter Y is defined in terms of the important requirements and the desired value obtained from the problem statement As seen previously, neither the definition of Y nor its required value for a satisfactory design is unique However, this approach does allow one to follow the iterative scheme and to terminate the iteration when Y attains the desired value Yr to within a chosen tolerance level , i.e., |Y Yr| (5.12) Design and Optimization of Thermal Systems Design characterization parameter, Y 320 Initial design Final design Number of iterations, N FIGURE 5.12 Variation of a parameter Y chosen to represent the acceptability of the design as a function of the number of iterations N Figure 5.12 shows the variation of Y as the iteration proceeds for a typical design problem The value may go up or down locally However, it is possible to determine if the scheme is approaching convergence in the long run, if divergence would occur, or if the results are simply oscillating without convergence Such a design parameter or criterion can also be used to determine the rate of convergence of the iterative scheme and to develop schemes that would accelerate convergence Many of the ideas presented in Chapter on the iterative convergence of nonlinear equations are applicable Since each iteration is time-consuming for most practical thermal systems, it is important to reduce the number of iterations needed to obtain an acceptable design Also, design variables that are particularly difficult to change, such as geometry, are often held constant while other variables are altered for reaching an acceptable design A discussion of some of these aspects follows System Redesign In the iterative redesign procedure, a given design is evaluated in terms of the problem statement, and, if it is found to be unacceptable, the system is redesigned by varying the design variables, keeping the conceptual design unchanged This new design is again evaluated and the iterative process continued until a satisfactory design is obtained As discussed previously, a single important quantity or parameter representing several important aspects in the problem may be employed to follow the iteration and to terminate it when a convergence criterion such as that given by Equation (5.12) is satisfied We now wish to address how redesign is undertaken at each step of the iteration Redesign involves choosing different values of the design variables in the problem The various types of design variables that are of interest in typical thermal systems are Geometrical configuration Materials employed Acceptable Design of a Thermal System 321 Dimensions of various parts Characteristics of different components or devices used in the system The performance of the system also depends on the operating conditions, which may be varied to obtain different product and system characteristics and for optimizing the operation of the system However, in system design, we are largely interested in the hardware of the system and thus the listed design variables are considered for redesigning a system It is necessary to follow a systematic approach in varying the design variables for redesign Consider a simple household refrigerator The configuration, materials, dimensions, and specifications of the components such as the compressor and condenser can be changed to obtain a new design If all these are varied at each iterative step, it is hard to keep track of the progress made from one design to the next and to determine the effect of each variable on the system performance One way of approaching redesign is to keep most design variables unchanged while one variable or a set of variables is altered The geometrical configuration, materials, and dimensions may be kept constant while different compressors, condensers, etc., are considered Similarly, the dimensions of the interior region, wall thickness, and other dimensions may be varied while the remaining design variables are held constant The given constraints are invoked when any particular design variable is being changed or selected Of course, the design variables may not be independent and may have to be varied together For example, the condenser capacity and its surface area go together, linking the dimensions with the component specifications Let us consider the forced-air heat treatment oven discussed earlier and shown in Figure 2.28 Again, the geometry, materials, dimensions, and components, such as the heater and the fan, are the main design variables The geometry and materials are often picked based on information available from existing or similar systems The range of variation in these two parameters is generally limited by the application and by the availability and cost of materials For instance, the configuration may be determined by the fact that a high side opening is needed to insert the material to be heated Similarly, cost considerations may limit the material selection to steel and aluminum In any case, the configuration and materials may initially be chosen to comply with such considerations related to the application, using available information on similar systems As the design process advances, even the geometry and the materials may be varied if a satisfactory design is not obtained However, the initial efforts are directed at dimensions and components that may be altered relatively more easily and which have wide ranges of variation, limited largely by the constraints in the problem A schematic of such an approach, which considers different types of design variables with a predetermined priority, is shown in Figure 5.13, with components varied first, followed by dimensions, then by materials, and so on This priority is based on the designer’s expertise and is a good candidate for automation, as discussed in Chapter 11 Even when attention is focused on the dimensions, these may be varied one at a time in order to determine the resulting effects If the effect of varying a given ... circuitry 3 04 Design and Optimization of Thermal Systems Library of Previous Designs Any industry involved with the design of systems and equipment would generally develop many successful designs... , i.e., |Y Yr| (5. 12) Design and Optimization of Thermal Systems Design characterization parameter, Y 320 Initial design Final design Number of iterations, N FIGURE 5. 12 Variation of a parameter... profiles in an enclosed body of water for two inflow/outflow configurations at different values of dimensionless time 3 14 Design and Optimization of Thermal Systems Acceptable Design of a Thermal